The present invention is directed to soot free autothermal reforming (ATR) of hydrocarbon feed containing higher hydrocarbons.
In the autothermal reforming, combustion of hydrocarbon feed is carried out with substoichiometric amounts of oxygen by flame reactions in a burner combustion zone and, subsequently, steam reforming of the partially combusted feedstock in a fixed bed of steam reforming catalyst. Substoichiometric combustion of hydrocarbons leads disadvantageously to formation of soot. Soot formation may be avoided by using a specific burner design and through controlling the operating conditions of the ATR process. Soot is formed in the flame of an autothermal reactor within certain ranges of operating conditions. When the amount of steam relative to the other components send to the ATR reactor is under a critical value, soot is formed in the reacting feed. One such burner useful in ATR is described in U.S. Pat. No. 5,496,170. The limiting amount of steam can be expressed as the critical steam to carbon ratio, which is the molar feed flow rate of steam to the molar flow rate of carbon in the hydrocarbon feed. The hydrocarbon feedstocks can be in form of natural gas or hydrocarbon including LPG, butane, naphtha, etc. The molar flow rate of carbon is calculated as the molar flow rate of the hydrocarbon times the carbon contents of the hydrocarbon.
Examples of operation conditions, which do not result in soot formation, are summarized in a paper by Christensen and Primdahl (Hydrocarbon processing, March 1994, pages 39-46). Those conditions are shown in Table 1. The tests have been conducted in a pilot plant. Due to heat loss from the relatively small pilot unit, the adiabatic ATR exit temperature will be higher than the measured ATR exit temperature. This means that if a large unit, from which the heat loss is negligible, is subjected to the exact same operating conditions, the ATR exit temperature will be close to the adiabatic ATR exit temperature. The soot precursors are formed in the combustion zone of the ATR. Most of the heat loss occurs after the combustion zone. A subsequent heat loss cannot have any influence on the reactions in the combustion zone. The oxygen to carbon ratio (O2/C) is also shown in Table 1. The definition of this ratio is analogue to the steam to carbon ratio, however, with steam substituted by oxygen. The exit temperature from the ATR reactor can be calculated from the O2/C ratio, when the heat loss from the reactor is known.
Advantageously, the process is operated at low steam to carbon ratios, since a low ratio lowers the investment expenses for an ATR plant and reduces the necessary energy consumption in operating the plant. Additionally, a low steam to carbon ratio makes it possible to optimize the produced synthesis gas composition for production of CO-rich gases for e.g. methanol or dimethyl ether synthesis and Fischer-Tropsch processes.
It has been found that arranging a low temperature reforming reactor with steam reforming catalyst upstream the autothermal reformer reduces the critical steam to carbon ratio. The steam reforming reactor removes or reduces the content of higher hydrocarbons. Higher hydrocarbons are hydrocarbons that consist of two or more carbon atoms. The steam reforming catalyst will convert the higher hydrocarbons to a mixture of methane, carbon monoxide, carbondioxide and hydrogen.
The low temperature steam reforming reactor can be an adiabatic prereformer or a heated prereformer, e.g. in form of a catalyzed heat exchanger coil as described in European Patent Publication No. 855,366. The exit temperature of an effluent stream from the low temperature steam reforming reactor is not higher than 600xc2x0 C.
Accordingly, this invention provides a process for the preparation of a hydrogen and/or carbon monoxide rich gas by catalytic autothermal reforming of a hydrocarbon feedstock in an autothermal reformer comprising further steps of:
(a) passing the hydrocarbon feedstock through a first reactor containing steam reforming catalyst to substantially remove the content of higher hydrocarbons in the hydrocarbon feedstock;
(b) passing the effluent from the first reactor to an autothermal reformer; and
(c) withdrawing from the autothermal reformer a product gas rich in hydrogen and carbon monoxide.
As an advantage of the invention it will be possible to operate the autothermal reactor at lower steam to carbon ratio than when using the unconverted feed stream as feed for the autothermal reformer.
As a second advantage, it is possible to preheat the feed gas for the ATR to a higher temperature when it contains no higher hydrocarbon. This results in a lower oxygen consumption by the process.
The invention is in particular useful in the steam reforming of accociated gas. Associated gas is natural gas produced during oil production. This gas is usually flared at the drilling rig, since it is not economical to ship associated gas. Associated gas has a high content of higher hydrocarbons.